Optical coherence analysis and specifically optical coherence tomography (“OCT”) are important medical imaging tools that use light to capture three-dimensional images in micrometer-resolution non-invasively from the sub-surface of a sample, such as a biological tissue. OCT is useful for such applications as industrial inspection and in vivo analysis of biological tissues and organs.
A common OCT technique is Fourier domain OCT (“FD-OCT”), of which there are generally two types: Spectral Domain OCT and Swept Source OCT. In both systems, optical waves are reflected from an object or sample. These waves are referred to as OCT interference signals, or simply as interference signals. A computer produces images of two-dimensional cross sections or three-dimensional volume renderings of the sample by using information on how the waves are changed upon reflection. Spectral Domain OCT and Swept Source OCT systems differ, however, in the type of optical source that they each utilize and how the interference signals are detected.
Spectral Domain OCT systems use a broadband optical source and a spectrally resolving detector system to determine the different spectral components in a single axial scan (“A-scan”) of the sample. Thus, spectral Domain OCT systems usually decode the spectral components of an interference signal by spatial separation. As a result, the detector system is typically complex, as it must detect the wavelengths of all optical signals in the scan range simultaneously, and then convert them to a corresponding interference dataset. This affects the speed and performance of Spectral Domain OCT systems.
In contrast, Swept Source OCT systems encode spectral components in time, not by spatial separation. Swept Source OCT systems typically utilize wavelength (frequency) swept sources that “sweep” in the scan range. The interference signals are then typically detected by a non-spectrally resolving detector or specifically a balanced detector system.
Compared to Spectral Domain OCT technology, Swept Source OCT often does not suffer from inherent sensitivity degradation at longer imaging depths, provides faster scanning speed and improved signal to noise ratio (“SNR”), and reduces the complexity of the detector system.
Despite the advantages of Swept Source OCT, certain problems exist. For example, large amounts of memory and processing power are required for resampling algorithms that include Fast Fourier Transforms (FFT), especially when real-time processing is desired. Selection of FFTs typically involves a cost tradeoff between core size/number of FFT points and the time required to perform the transform, also known as the transform time. The transform time increases with increasing interference dataset width. As a result, operators typically purchase different versions of the OCT equipment in response to their signal processing needs and dataset width, which increases cost. The processing burden of the computer systems that perform the resampling increases as the number of sample points generated per scan of the sample increases. The performance of general purpose processors is insufficient for the needs of real-time OCT data acquisition and imaging.